1. Field
This disclosure is concerned generally with porous inorganic support materials useful for the immobilization of enzymes and specifically with methods of regenerating certain such carriers which have been found useful for the immobilization of glucose isomerase.
2. Prior Art
In patent applications Ser. No. 332,807 and Ser. No. 332,739, cited above, there are disclosed methods of immobilizing glucose isomerase within the pores of high surface area, highly porous alumina particles to provide a very efficient and reuseable immobilized glucose isomerase system useful for the isomerization of glucose to fructose. As described in those patent applications, it was found that the porous alumina carrier, preferably in particulate form (e.g. within 4-200 mesh), should have an average pore diameter at least as large as the enzyme, but less than about 1000A, preferably less than 500A or between about 100A and 500A. In patent application Ser. No. 507,209, cited above, and filed of even date with this application, an improvement over the alumina carrier is disclosed. The improved glucose isomerase carriers have incorporated thereinto varying amounts of magnesia, the preferred carriers consisting of both alumina and magnesia with the magnesia preferably constituting about 0.84 to 12.0% by weight. Such carriers are referred to herein as MgO-Al.sub.2 O.sub.3 carriers to distinguish them from the Al.sub.2 O.sub.3 carriers.
In using either porous Al.sub.2 O.sub.3 or porous MgO-Al.sub.2 O.sub.3 carriers for the adsorption and, hence, immobilization of glucose isomerase, it has been found that the resulting composites demonstrate a high degree of stability and relatively long enzymatic half-lives. These qualities make the composites commercially attractive since such characteristics are desirable for any large scale conversion of glucose-containing solutions to fructose-containing solutions. The desirability of being able to continuously and economically convert glucose to fructose is well recognized, especially via enzymatic isomerization methods.
Even though the above-described porous Al.sub.2 O.sub.3 and porous MgO-Al.sub.2 O.sub.3 carriers can be used to prepare immobilized glucose isomerase compositions having relatively long half-lives, the use, especially the continuous use, of such composites is economically time-limited. Regardless of the length of enzymatic half-life of the composites, it can be appreciated that the total enzymatic activity tends to decline with time. Thus, at a given point in time, it becomes uneconomical to continue using the composites because of reduced activity. Accordingly, at that time it becomes more economical to simply replace the spent composite with fresh composite.
Although the above-described porous carriers are relatively inexpensive and can be discarded after use without detracting significantly from the overall favorable economics of using such carriers for glucose isomerase, it would be highly desirable if the carriers could be regenerated for reuse. Carrier reuse would not only permit yet further economies, but also avoid problems associated with discharge of the spent composites. It is known that various pyrolysis treatments can be used to burn off organic constituents on inorganic materials. We have found, however, that a relatively simple pyrolysis step per se is not sufficient to prepare the porous carrier for immediate and economical reuse because the pyrolyzed carriers still have associated therewith various contaminants (e.g., various metal ions from the substrate) which tend to minimize subsequent enzyme loading and/or half-life. Quite surprisingly, we have found that the carriers can be readily regenerated for economical reuse by a controlled pyrolytic step followed by treatment with a solution which appears to remove undesirable metal ions associated with the carriers after pyrolysis. Our two-step method of regenerating the carriers is described in detail below and this disclosure is directed specifically toward methods of regenerating the MgO-Al.sub.2 O.sub.3 carriers disclosed in patent application Ser. No. 507,209, filed herewith in the names of D. L. Eaton et al.